A conventional camera lens tends to gather more light at the center than at the edge. This results in a variation in the light intensity from the center to the edge of an image transmitted by a lens when viewing a uniformly illuminated object. In the application of a line scan camera and associated lens in a system for the line-by-line scanning of a uniformly illuminated document, the intensity of the scan line will typically vary as much as 2:1 across the image developed by the lens. This object lens aberration is called "vignetting". The variation in image light intensity caused by vignetting results in distortion, or an inaccurate representation of data mark or character information viewed during the scan line of a document. In the application of the line scan system for reading marks on a student answer sheet, the "white-to-black" response for a mark in the center of the sheet would be larger than than for the same mark on the edge of the sheet.
Variations of light intensity with respect to document ambient or background can also be the result of the non-uniformity of the grain of the document paper, non-uniformity of the illumination of the document, dirt on mirrors or other surfaces.